European Patent Publication No. 291217 describes a system primarily aimed at vehicles having electronic anti-lock braking systems although it may be used as a stand alone system.
Two versions are disclosed one of which uses a multi-pulse signal generating device means at each wheel. The other is a single-pulse per wheel revolution generator. Further details of these systems are disclosed in the publications for co-pending European Patent Applications No. 91300939.9 and 91300938.7 both dated 5th February 1991.
The multi-pulse signal based unit of the above described systems have been found to have a minimum speed above which the system operates satisfactorily of about 10 km/h when a wheel speed generator of 96 teeth is used. In the case of a 48 tooth generator the minimum speed is of the order of 18 km/h. Thus multi-pulse systems are restricted from operating satisfactorily at low speeds. By contrast a single pulse per revolution system, which is also described in the earlier applications and is based on time per revolution, has been found to operate well at low speeds but at high speeds, where the time periods to be measured are very short, it is necessary to use comparatively expensive computer systems and thus the system is not financially efficient.